Multiple degree of freedom substrate manipulator

ABSTRACT

A system for manipulating a planar substrate such as a semiconductor wafer is provided. The manipulator is typically used in conjunction with an XY stage to focus and planarize a wafer with respect to a tool. The manipulator employs redundant actuators of different types and a control system that uses low-bandwidth, high efficiency actuators to provide low frequency forces and high-bandwidth, but less efficient, actuators to provide all other forces. The manipulator provides support and manipulation of a substrate while minimizing errors due to thermal distortion.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] This application relates to handling substrates, and moreparticularly to handling of a substrate with control over an operatingrange spanning a minimum of three and a maximum of six degrees offreedom.

[0003] 2. Description of Related Art

[0004] Substrate handling mechanisms are often used in equipmentdesigned to process semiconductor wafers, flat screen liquid crystaldisplays, printed circuit boards, and micromachine assemblies. Similarmechanisms are used in failure analysis systems, electrical andfunctional testing systems, and IC packaging systems.

[0005] Modern semiconductor wafers may be cylindrical substrates ofsilicon, up to 300 mm in diameter, and may be less than 1 mm thick.During many of the manufacturing processes of semiconductor devices,wafers are held on a substrate holder known as a chuck, using vacuum.The chuck is often also used as a substrate handler, to position thesubstrate at a specified location in up to six dimensions, and to movethe substrate from an input section of a processing or testing system,through the process steps, and finally to an output section for removal.A chuck may be machined from aluminum, silicon carbide or othermaterial, having a top surface that is machined to be flat. There may bevacuum outlets in the flat surface of the chuck that may be in the formof connected grooves cut into the flat surface. The vacuum system holdsthe wafer, or other planar substrates such as liquid crystal displaypanels, to the flat chuck surface. A non flat substrate or wafer may bemade flatter by the action of the vacuum hold down of the chuck. On theother hand, the strain placed upon the chuck by the flattening action ofthe vacuum may result in warpage of the chuck surface, and consequentloss of planarity of the wafer or substrate. One method for addressingthe chuck warpage problem is to make the chuck thicker and more massive,and thus more resistant to the strain of the wafer. However, increasingthe mass of the chuck results in increased force necessary to move thechuck, and consequently increases stage mass and motor power levels.

[0006] Many of the manufacturing steps used to create integratedcircuits, and other small dimension devices on substrates, require thatthe wafer or substrate position and orientation be precisely controlled.This requirement may be meet using what is known as an X Y stage tomanipulate the wafer over a planar region, and what is known as aZ-theta chuck system to raise and lower and rotate the wafer about anaxis normal to the nominal XY plane. Certain processes also requireactive manipulation of the plane of the wafer in order to maintain thewafer surface parallel to the plane of the process tool. This may benecessary if the wafer front surface and the wafer back surface are notexactly parallel. This condition is known as taper. The thickness of thewafer may also vary from place to place, a situation known as bow. Thus,a chuck may be required to rotate the front surface of the wafer in whatis known as roll, pitch and yaw. Each of these three motions can beconsidered to be rotations around the X, Y or Z axis respectively.

[0007] Processes that require these sorts of motions include step andrepeat camera imaging systems, which need the front surface of the waferto be flat over a large surface area. If a change in the front surfacelocation with respect to the focal plane of the camera occurs during anyof the rotations around the orthogonal axes, then the resulting imagewill not be in focus at all points.

[0008] As semiconductor technology has increased with improvedsemiconductor performance, the wafer diameters have increased oversuccessive generations of semiconductor manufacturing equipment fromless than 100 mm to the current standard of 300 mm. At the same time,the precision requirements of the semiconductor manufacturing equipmenthas become tighter as the critical line width sizes have become smallerwith the increased technological level. The requirement to maintaintighter line widths that accompanies the increase in technologicallevel, also results in increased alignment accuracy and precisionrequirements, and to a decreased depth of field capability. The depth offield problem requires that the wafer surface be flatter, whichconsequently requires that the wafer chuck be flatter and strong enoughto hold the wafer flat. The increase in required precision thus includesthe need for improved capabilities to move the wafer accurately in thehorizontal plane, the XY plane, as well as in the vertical direction,i.e. Z. Increase in required precision also requires accurate motion ofthe wafer in the roll, pitch and yaw directions.

[0009] Traditional chuck systems rely on mechanical bearings andmachining tolerances to maintain the plane of the wafer attached to thechuck parallel to the XY plane of the stage. Mechanical approaches to awafer chuck become more difficult as the mass of the chuck increases andthe precision requirements become more severe. This is because as thewafer chuck mass increases, the mechanical bearings used to constrainthe chuck necessarily become larger. As the precision requirementsincrease, the mechanical bearings must resort to increased levels ofwhat is known as a preload in order to achieve the necessary stiffnessto maintain precision and avoid vibration. As each of the elementsbecomes more massive and the stiffness increases, the forces required tosupport the more massive chuck and overcome the friction of bearings andactuators also consequently increases. Typical electromechanicalactuators, such as motors, dissipate power in proportion to the squareof the force they produce. Thus, as the chuck mass increases and thebearing mass increases, the size of the actuator and the actuator powermust also increase, resulting in increased power dissipation and localheating of the wafer. Heating of the wafer may be a problem becauseexpansion of the wafer results in a shifting of the location ofdifferent parts of the wafer, and thus loss of precision andrepeatability. The different coefficients of thermal expansion of thealuminum (or other material) chuck and the semiconductor wafer may alsoresult in mismatched stress between the chuck and the wafer, and mayresult in wafer warpage. Thus, power dissipation in the actuators of thechuck may lead to thermal gradients and corresponding changes in themechanical dimensions of the chuck mechanism, which may be another majorimpediment to achieving the levels of precision required in manysemiconductor processes, liquid crystal display processes, thin filmmagnetic head processes, and micro-machining processes.

[0010] Some of the above described problems have been addressed in priorart chuck mechanisms by restricting the range of Z motion to less than0.1 mm, and using a flexure suspended chuck driven in the Z direction bypiezo actuators. While these devices provide large forces withnegligible heat generation, they are unable to provide sufficient rangeof motion to allow wafer transfers between a transfer robot and thechuck. This is because during the loading and unloading of a wafer ontothe chuck, a minimum gap of approximately 6 mm must be establishedbetween the bottom of the wafer and the top of the chuck. This spacingis necessary for the robot or operator to insert a vacuum paddle betweenthe chuck and the wafer to move the wafer while only touching the waferbackside, and thus prevent damage to the front surface of the wafer.Contact with the front surface of the wafer may result in physicaldamage such as scratching, and may also result in contamination of thedevices on the front surface. Thus, piezo actuators must have a separatemechanism to provide the chuck with enough separation to allow wafertransfers. This additional requirement of piezo actuators increases thecost, complexity, and the mass of the stage.

[0011] Another problem with mechanical methods of moving a wafer chuckaround, such as the piezo actuators, is that the physical contact of thepiezo actuators with the chuck may represent another source of stressand strain, and therefore cause deformation of the chuck and resultingloss of precision. As noted before, certain manufacturing processes,such as semiconductor device manufacture, liquid crystal displaymanufacture, and thin film magnetic head manufacture, require extremeflatness in the manufactured device, and consequently extreme flatnessin the chuck.

[0012] Since the chuck mechanism is often carried on an XY stage, thechuck design can have a major influence on the XY stage design andperformance. The XY stage performance is affected by the mass and heightof the chuck. As the chuck mass increases, the stage must be made largerand must be able to dissipate more power and heat in order to achieve asufficient level of performance and precision. Since the powerdissipation increases with the square of the total stage mass, a smallincrease in mass may result in a large increase in power consumption.The heat dissipated by the stage influences the stage accuracy andprecision due to thermal disturbance of the air inside a tool, since thethermal gradients disrupt the precision of the distance measuring laserinterferometers that are typically used to measure the location of thestage. Changes in heat dissipation also affect the precision of theinterferometer, and thus the repeatability of the operation. As anexample, the operation of a step and repeat camera requires the movementof the wafer chuck from image field to image field be as precise aspossible in order to minimize the amount of time spent on each imagefield in fine alignment.

[0013] Many semiconductor process tools involve optical lens elementsthat must be in close proximity to the top of the wafer surface. Theclearance between the top of the wafer surface and the process tools,for example in a step and repeat camera, or a direct write laser beamlithography tool, may be less than 1 mm. In the specific case of waferprobing, actual mechanical contact is made between the wafer and probepins. Thus, in order to achieve the necessary clearance for wafertransfers, i.e. >6 mm, the wafer must be raised or the chuck must belowered. Since the separation of wafer and processing tool maybe lessthan 1 mm, in many such cases the wafer cannot be raised, and thus thechuck must be lowered. However it has been previously noted that it isdifficult to have high precision and large motion simultaneously. Thusprior art chuck systems have the XY stage carrying the chuck move awayfrom underneath the tool before wafer transfers occur. This causesincreased time, cost, size and heat dissipation of the stage.

[0014] The trend in the various fabrication industries, such assemiconductor, liquid crystal display, thin film magnetic head, andmicro-machining, toward larger substrate sizes and increased precisionin chuck location, results in the need for a chuck that can provideprecision control in X, Y, Z, roll, pitch, and yaw. The chuck must alsoprovide a large motion in the Z direction, minimize the mass and powerneeded to move the chuck on the XY stage, maintain a constanttemperature at the chuck and wafer, and not apply stress to the wafer.The chuck must also be able to compensate for externally applied forceson the wafer, such as found in wafer probers.

SUMMARY OF THE INVENTION

[0015] According to the present invention, an apparatus for manipulationof a planar substrate includes a housing movable on an X-Y stage thattransports the housing in at least two orthogonal directions, a surfacehandler disposed within the housing and having a substantially flatsurface, a plurality of position sensors disposed about at least one ofa periphery of the surface handler and a periphery of the housing, atleast one air-bearing sleeve disposed in the housing, at least onepiston disposed within the at least one sleeve and having at least onepressure chamber formed by the piston and air-bearing sleeve, at leastone valve that may be used to modulate the flow of fluid from thepressure chamber to an exhaust pressure, at least one voice coil motordisposed within the piston, at least one air-bearing pad disposed at oneend of the piston opposite the pressure chamber and acting against asurface of the surface handler opposite the flat surface where thepressure from the air-bearing pad against the surface handler is opposedby a magnetic attraction between the surface handler and the piston, aplurality of magnetic regions located at preselected portions of thesurface handler, a plurality of radial actuators disposed on the housingand corresponding to at least a portion of the plurality of magneticregions of the surface handler, and a plurality of tangential actuatorsthat correspond to at least a portion of the magnetic regions of thesurface handler.

[0016] The housing may move the surface handler in at least an Xdirection, a Y direction, a Z direction, a yaw direction, a rolldirection and a pitch direction. The housing may include at least oneactuator for each direction of motion. The planar surface handler may becompletely supported and transported by the actuators. The surfacehandler may be a chuck. The chuck may be a vacuum chuck. The chuck maybe an anodized aluminum alloy circular cylinder. The chuck may have acentral region having an array of grooves embedded in the flat surfaceand connected to a vacuum control line. The central region may be acircle having a radius of approximately 200 mm, and the array of groovesmay be a plurality of concentric circular grooves having a common centerat approximately a center of the chuck. The chuck may further include aperipheral region having a second array of grooves embedded in the flatsurface and connected to a second vacuum control line. The second arrayof grooves may be an annular region of approximately 200 to 300 mmradius. The substantially flat surface of the surface handler mayinclude a plurality of vacuum conduits. The plurality of vacuum conduitsmay include at least two different vacuum control areas, each areahaving a separately controllable vacuum pressure source. The pluralityof vacuum conduits may be arranged in rings. The surface handler may notdirectly contact the housing. The plurality of position sensors may bethree. The plurality of position sensors may provide positionmeasurements in two dimensions. The surface handler may include aplurality of holes through which pins, fixedly attached to the housing,are inserted perpendicular to the flat surface of the surface handler,and, in response to a signal that lowers the surface handler withrespect to the housing, the pins may extend above the flat surface by atleast a predetermined distance, which may be greater than approximately6 mm. The plurality of holes may be three. The plurality of pins may behollow. The hollow pins may be connected to a vacuum line. The surfacehandler may have an approximately circular top surface. The magnet ofthe voice coil motor disposed within the piston may provide the sourceof magnetic flux that preloads the surface handler against an airbearing pad. The magnet may attract the ferromagnetic region of thesurface handler against the air bearing with a force sufficient toestablish a desired stiffness and corresponding flying-height betweenthe surface handler and the air-bearing pad disposed on top of thepiston. The piston may include an air-bearing pad disposed above thesurface of the piston and allowed to pivot with respect to a verticalaxis of the piston. The pivot may be a ball pivot. The fluid in thesleeve may be air. The apparatus may also include a controller thatmodulates the valve to regulate the pressure in the pressure chamber andmodulates the force in the voice coil motor in order to adjust theheight of the surface handler with respect to the housing. The pistonmay have no sealing surface with respect to the sleeve to prevent thecompressed air from leaking out at a controlled rate. The controlledrate may be determined by relative diameters of the piston and thesleeve. The apparatus may also include a single valve connected to eachpressure chamber and normally operated in a partially open state toregulate the flow of fluid from the pressure chamber to an externalsource of pressure that is maintained at a pressure substantially belowthe nominal chamber pressure. The valve may be opened or closed inproportion to a signal from the controller in order to regulate the netmass flow of fluid between the fluid entering the chamber from theair-bearing sleeve and the mass flow of fluid leaving the chamberthrough the valve. The plurality of pistons may be three. The pistonsmay be driven differentially to generate motion around the X and Y axes.The controller may operate the valve and corresponding voice coil motorso as to minimize the power dissipated in each voice coil motor. Theplurality of magnetic regions of the surface handler may be three. Theplurality of tangential actuators may be three. The coils of thetangential actuators may be disposed in a portion of the housing and thelow-reluctance components of the actuators may be fixed in the surfacehandler. The surface handler may have a plurality of ferromagneticsurfaces disposed around a peripheral portion of the surface handlerthat interact with a plurality of radially-acting electromagneticactuators disposed in the housing. The plurality of actuators may bethree. The surface handler may further include a plurality ofprojections disposed in a symmetric pattern on the surface opposite theflat surface and wherein the housing includes a plurality of depressionsdisposed in a corresponding pattern. The plurality of projections mayequal three. The plurality of projections may further include balls andthe plurality of depressions include vee-grooves. The apparatus mayfurther include a control system that controls the plurality of radialactuators in a differential mode where the total amount of powerprovided to all of the radial actuators is maintained at a substantiallyconstant value while generating net forces between the housing andsurface handler in the X and Y plane. The housing may further include arecessed bottom portion disposed to fit into a recess in a correspondingX-Y stage.

[0017] According further to the present invention, an apparatus formanipulation of a substrate includes a handler having a flat surfacethat holds a substrate, a housing that holds the handler, a plurality oftangential actuators that cause rotation of the handler about an axisperpendicular to the surface of the handler, and a plurality of radialactuators, each actuator moving the handler with respect to the housingin a selected radial direction independent of rotation caused by thetangential actuators, wherein an increase in the force level of one ofthe plurality of radial actuators is counterbalanced by a reduction inthe force level of at least one other of the radial actuators so thatthe power consumed by all of the radial actuators together issubstantially constant.

[0018] According further to the present invention, an apparatus formanipulation of a substrate includes a handler that holds a substrate, ahousing having a bottom surface that holds the handler, and a pluralityof vertical actuators disposed in the housing to move the handler towardand away from the bottom surface of the housing, wherein an increase indistance from the bottom surface to the handler provided by one of thevertical actuators is counterbalanced by a decrease in distance from thehandler to the bottom surface by another one of the vertical actuatorsto provide a rotation about an axis of rotation perpendicular to adirection of motion provided by the vertical actuators, wherein the axisof rotation is disposed at a location corresponding to a location in asubstrate provided on the handler. The vertical actuators may include amagnet that attracts a magnetic plate disposed on the bottom surface ofthe handler and an air bearing that pushes against the plate. Thehandler may rotate in a plane that is substantially perpendicular to thedirection of motion provided by the vertical actuators.

[0019] According further to the present invention, an apparatus formanipulation of a substrate includes a handler that holds the substrate,a housing that holds the handler, a plurality of patterned surfacesdisposed on one of: the housing and the handler, wherein the patternedsurfaces include one of the following combinations: at least six lineararrays, at least four linear arrays and at least one grid array, atleast two linear arrays and at least two grid arrays, or at least threegrid arrays, and a plurality of readers that read the combination todetermine a location of the substrate in six degrees of freedom. Thereaders may be optical encoders.

[0020] According further to the present invention, an apparatus formanipulation of a substrate includes a handler that holds the substrate,a housing that holds the handler, and a plurality of actuators that movethe handler in response to at least one control signal, where theplurality of actuators further includes at least one high frequencyactuator and at least one low frequency actuator that provide motion ina single direction and wherein the at least one control signal includesa high frequency portion that is provided to the at least one highfrequency actuator and a low frequency portion that is provided to theat least one low frequency actuator. The at least one low frequencyactuator may reduce static forces on the at least one high frequencyactuator. The at least one low frequency actuators may include at leastone radial electromagnet. The at least one high frequency actuator mayinclude at least one tangential electromagnet. The at least one lowfrequency actuator may be a pneumatic actuator. The pneumatic actuatormay actuate the handler in a vertical direction. The at least one highfrequency actuator may include an electromagnetic actuator that providesa force in a vertical direction on the handler.

[0021] According further to the present invention, an apparatus formanipulation of a substrate includes a handler that holds the substrate,a housing that holds the handler, and a plurality of vertical actuatorsdisposed in the housing to move the handler toward and away from thebottom surface of the housing, wherein the vertical actuators are drivendifferentially to counteract and compensate for any vertical off axisforces applied to the substrate. The vertical actuators may include amagnet that attracts a magnetic plate disposed on a bottom surface ofthe handler and an air bearing that pushes against the plate. Theapparatus may further include a ball disposed in each vertical actuatorand a corresponding vee-groove disposed in the handler to allow roll andpitch motion of the handler. Compensation for vertical off axis forcesmay be provided primarily by a pneumatic actuator.

[0022] According further to the present invention, an actuator includesa piston movably guided by a fluid-bearing, a fluid chamber formed atone end of the piston having a chamber pressure controlled by balancinga fluid flowing into the fluid chamber with fluid exiting the chamberthrough a controllable orifice, and a voice coil motor disposed to movethe piston located within the confines of the piston. The actuator mayalso include a control system that supplies large low-frequency forcesby modulating the chamber pressure, and supplies low amplitudehigh-frequency forces via the voice coil motor.

[0023] According further to the present invention, an actuator thatmanipulates an object includes at least three tangentially acting voicecoil motors disposed to move the object relative to the housing in a yawdirection, at least three radially acting electromagnet actuatorsmaintaining a substantially constant gap between the object and ahousing independent of motion in the yaw direction, and a control systemthat supplies the object with large low-frequency forces in a plane ofthe object using the radially acting electromagnet actuators, andsupplies low amplitude high-frequency forces in the plane of the objectand provides forces for motion in the yaw direction using the tangentialvoice coil motors.

[0024] According further to the present invention, a substratemanipulator providing six degrees of controllable motion for an objectincludes a housing, a substrate handler that holds the object and doesnot contact the housing, at least three actuators that move the objectin Z, Roll and Pitch directions, and at least three actuators that movethe object in X, Y and Yaw directions.

[0025] According further to the present invention, a substratemanipulator providing control of movements of an object in the Z, rolland pitch directions includes a housing, a substrate handler that holdsthe object disposed in the housing, and at least three actuators thatmove the object in a Z direction.

[0026] According further to the present invention, a substratemanipulator providing control of movements in the Z, roll, pitch and yawdirections, includes a housing, a surface handler disposed in thehousing, at least one hub, disposed concentric to a center point of thehousing and free to rotate about the Z axis using at least threemagnetically preloaded air bearing regions located between the housingand the hub, a pair of magnetically preloaded bearings that constrainthe hub to rotate substantially about the axis of the housing, at leastthree tangential voice coil motors that provide a yaw moment withoutcreating any substantial XY force on the hub relative to the housing,and at least three Z actuators to move the surface handler with respectto the hub.

[0027] The size and configuration of the pneumatic actuator may beselected to enhance actuator performance in precision applications. Thediameter of the air bearing sleeve affects the vertical actuatorperformance in three aspects.

[0028] The stiffness of the air bearing to piston assembly is influencedby the operating pressure and circumference of the air-bearing sleeve. Alarger sleeve will yield greater stiffness for the same fluid pressuresupplied to the air bearing. Greater stiffness is preferred in order toenhance precision of the manipulator.

[0029] A larger diameter air bearing sleeve implies a larger effectivearea of the pneumatic piston. For substantially similar net verticalforce a larger piston area will require lower pressures in the pressurechamber. Generally one avoids low pressures in a pressure controlsituation due to the somewhat larger valve opening that is required toachieve a desired mass flow rate. In the present invention, the lowchamber pressure is an advantage in that it effectively isolates thechamber pressure from the air-bearing performance. For an examplecylinder diameter of approximately 50 mm, a typical working pressure inthe pressure chamber is between 0 and 0.05 bar (that is between 0 and 5%of 1 atmosphere) above atmospheric pressure. Such low operatingpressures ensure that the air bearing is unaffected by the minutechanges in chamber pressure experienced during normal operations. Thedegree of effective isolation between the chamber and the air bearingensures that the mass flow of fluid into the chamber is nearly constantin normal operation.

[0030] The low chamber pressure and therefore constant mass flow offluid into the chamber from the air bearing provide an extremely “quiet”source of chamber pressure. A well known problem with fluid controlsystems is the level of noise that is often injected in to the pressurechamber due to turbulence in the servo valve. This is particularly truewhen a servo valve is used to control the rate of fluid entering thechamber. As the fluid passes across the control orifice in the valve,the fluid may enter the turbulent regime (Reynolds number above 2000).The turbulence in the fluid creates minute pressure disturbances thatmay be apparent as a source of noise making nanometer-level of precisiondifficult to achieve. The present invention avoids the use of an“orifice” sourcing fluid into the chamber. The air gap between thepiston and air bearing sleeve are sufficient to ensure that the fluidentering the pressure chamber exhibits laminar flow. This avoids the“noise” of turbulent flow. The servo valve placed on the exhaust with anexhaust pressure to a pressure “source” held substantially below thepressure in the chamber (ie: a source of vacuum) ensures that theexhaust valve will be operated in the turbulent regime. Since theturbulence is on the exhaust side of the pressure chamber, the “noise”associated with the turbulent flow in the exhaust valve does notinfluence the pressure chamber. Furthermore, it will be readily apparentto those skilled in the art of compressible fluid flow, that operatingthe exhaust valve in the turbulent regime (above Reynolds number of2000) ensures that no pressure variations in the exhaust pressure (ie:variation in the vacuum pressure) will propagate across the servo valveand influence either the chamber pressure or the mass flow through theservo valve. This allows a less expensive vacuum pump to be used.

BRIEF DESCRIPTION OF DRAWINGS

[0031]FIG. 1 shows an isometric view of a housing and a chuck accordingto the system described herein incorporated on an XY stage;

[0032]FIG. 2 shows a top view of the housing and the chuck along with aconfiguration for vacuum attachment according to the system describedherein;

[0033]FIG. 3 shows an isometric of the housing and the chuck accordingto the system described herein;

[0034]FIG. 4 shows a different isometric of the housing and the chuckaccording to the system described herein;

[0035]FIGS. 5A, 5B, and 5C show a cross section view of the housing andthe chuck according to the system described herein;

[0036]FIG. 6 shows an isometric view of the housing with the chuckremoved according to the system described herein;

[0037]FIGS. 7A and 7B show a different isometric view of part of thehousing and the chuck according to the system described herein;

[0038]FIG. 8 shows an isometric view of the bottom of the chuckaccording to the system described herein;

[0039]FIGS. 9A and 9B show a cross section of the housing and chuckaccording to the system described herein;

[0040]FIGS. 10A and 10B show a top view of the housing with the chuckremoved according to the system described herein;

[0041]FIG. 11 shows an isometric view of an alternative embodiment ofthe housing and chuck according to the system described herein;

[0042]FIG. 12 shows a different isometric view of an alternativeembodiment of the housing and chuck according to the system describedherein;

[0043]FIG. 13 shows an exploded isometric view of an alternativeembodiment of the housing and chuck system described herein;

[0044]FIG. 14 shows an exploded isometric view of a portion of analternative embodiment of the housing and chuck system described herein;

[0045]FIG. 15 shows an isometric view from the underside of analternative embodiment of the housing described herein;

[0046]FIGS. 16A, 16B, and 16C show an isometric view from the undersideof an alternative embodiment of the housing and chuck system describedherein;

[0047]FIGS. 17A, 17B, 17C, 17D, and 17E show a cross section of analternative embodiment of the housing and chuck system described herein;

[0048]FIGS. 18A, 18B, and 18C shows an isometric view of the chuck of analternative embodiment of the housing and chuck system described herein;

[0049]FIG. 19 shows a block diagram of a chuck controller according tothe system described herein;

[0050]FIG. 20 shows a block diagram of a radial, tangential and verticalforce controller according to the system described herein;

[0051]FIG. 21 shows a block diagram of a radial force controlleraccording to the system described herein;

[0052]FIG. 22 shows a block diagram of a radial motor controlleraccording to the system described herein;

[0053]FIG. 23 shows a block diagram of a tangential motor controlleraccording to the system described herein;

[0054]FIG. 24 shows a block diagram of a Z actuator force controlleraccording to the system described herein;

[0055]FIG. 25 shows a block diagram of the pressure controller accordingto the system described herein; and

[0056]FIG. 26 shows a diagram of a pressure observer circuit accordingto the system described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

[0057] With reference to FIG. 1, shown is an isometric view of an XYstage 1 with the chuck and housing mechanism 2 mounted on the stage. Thebasic elements of the XY stage would be easily recognized by thoseskilled in the art as the dual Y-axis drives 4, a single X axis drive 5which together drive the slider 6 and whose position is typicallymonitored by an interferometer (not shown) that monitors slider positionvia L-shaped interferometer mirror 3. The stage is not part of thepresent invention. It is shown merely for illustrative purposes.

[0058] With reference to FIG. 2, shown is a top view of the substrateholder 10, supported by a housing 12 which substantially surrounds thesides and bottom of the substrate holder 10, and carries the substrateholder 10 in a plane on an XY stage 1. The substrate holder 10 has threewafer or substrate lift pins 14, each of which has a central hollow forconnection to a vacuum pressure line for securing the wafer or substrateto the pins when the surface handler is lowered for substrate exchanges.The substrate holder 10, also known as a chuck, is suspended withoutcontact to the housing 12. The housing 12, is connected to the XY stagefor large-scale motion in the X and Y directions, by means of threeflexure mounts 16 in this example embodiment. Each position sensordetector 18 may provide a separate horizontal and vertical positionmeasurement for a six measurement system. Alternative embodiments mayincorporate different type and number of position sensors and maymonitor a minimum of 3 and maximum of 6 degrees of freedom of motion ofthe substrate holder relative to the housing. A Cartesian coordinatesystem incorporating an X axis 7, Y axis 8 and Z axis 9 withcorresponding rotations of roll, pitch and yaw around the 3 axesrespectively is indicated in FIG. 1.

[0059] With reference to FIGS. 2 and 3, the substrate holder or chuck 10is shown with the three lift pins 14, which may be selectively connectedto a vacuum source to hold the wafer or other planar substrate duringwafer exchanges, for example during wafer loading and unloadingoperations. Additional vacuum attachment methods are used to hold thewafer or other planar substrate onto the holder 10 when the holder 10 israised (normal operation). Shown is a two section vacuum system havingroughly circular rings of vacuum. The shown two section vacuum systemarrangement may be used for chuck systems that have variable diametersubstrates to hold down to the chuck, or for situations where differentvacuum pressures on the outer edge of the substrate may be required,such as for holding slightly warped wafers. FIG. 2 shows an inner vacuumregion and an outer vacuum annulus. A land 11 provides a seal againstthe substrate bottom surface that separates the inner vacuum region fromthe outer vacuum annulus. A second land 13 provides a seal against thesubstrate bottom surface to isolate the outer vacuum annulus at the edgeof the substrate. The vacuum regions may be patterned with miniaturegrooves, pyramids, serpentine grooves or other vacuum channel shapesknown to those skilled in the art. Note that, in some embodiments, theconnections between the various ones of the sets of vacuum rings may notbe visible on the surface of the chuck as is shown in FIG. 2.

[0060] The chuck 10 may move in the X and Y axis directions for smalldistances within the housing 12. Such radial movements may be used, forexample, in the fine alignment of an optical field in a step and repeatexposure system, after the XY stage has brought the proper portion ofthe substrate or wafer to the approximate position. Such movement allowsrotation to be performed around a focal plane.

[0061] The radial motion of the chuck 10, within the housing 12 iscontrolled by three radial electromagnet coils 20 in this illustrativeembodiment, which are embedded in the housing 12, and act on threeferromagnetic surfaces embedded in the chuck 10. The only power thatneeds to be generated in this radial motion is the electrical currentused in the electromagnet coils 20, which are in the housing 12, and notconnected directly to the chuck 10, and thus the heat generated duringthe XY motion does not impact the chuck as much as if theelectromagnetic coils were located in the chuck. As discussed above, anincrease in temperature will result in the chuck 10 expanding, and thusshift the wafer position.

[0062] Even though the heat producing portion of the radial motionactuators is separate from the chuck, there still may be some thermalcontact that adversely effects the temperature of the chuck and theattached wafer, especially during periods when the amount of heatgenerated increases, such as at the beginning of an exposure sequence ina step and repeat camera imaging system. To address such a source ofunwanted heat, the three radial electromagnet coils 20 move the chuck bymeans of differential magnetic fields, wherein if one electromagneticcoil needs to increase the magnetic field to draw the chuck closer, acombination of the other two electromagnetic coils reduce their fieldstrengths to compensate for the increased power consumption, resultingin overall constant power consumption for the radial motion of thechuck. Thus the power consumption, and consequent heating, is designedto be held constant irrespective of the amount of motion, and thepower/heat generators are not in direct contact with the chuck orsubstrate. It is clear that many different types of motion actuators maybe used. In general, electromagnet (i.e., EM) actuators have a powerconsumption that varies proportionally to the force, and voice coilmotors (i.e., VCM) are proportional to the square of the force.

[0063] The chuck 10 may also have sufficient motion in the Z direction(i.e., the vertical direction downward) to cause the chuck 10 to retractbelow the lift pins 14, thus effectively holding the wafer above thechuck surface for placement or removal by a transfer robot arm.Alternatively, the lift pins 14 may be flush against the surface of thechuck 10 or slightly recessed beneath the surface when the chuck 10 israised for normal tool operations. The chuck 10 vertical motion isprovided by a combination of three pistons and three Z electromagneticdevices in this illustrative embodiment, for example voice coil motors(i.e., VCM) which are discussed in more detail in connection withsubsequent figures. The vertical or Z motion needs to be sufficient forsubstrate handling on and off the chuck, and preferably is more than 6mm, and even more preferably is 11 mm.

[0064]FIG. 3 is an isometric view of the chuck 10, housing 12, and liftpins 14. The isometric view provides a better view of the orientation ofcertain features of the substrate handling system, especially when usedin conjunction with the schematic view of FIG. 2. FIG. 2 shows the chuck10 in the down position, where it is as deeply embedded as possible inthe housing 12, and thus the fixed lift pins 14 are at a full extensionposition above the surface of the chuck 10. The lift pins 14 may slidevertically within close fitting bores in the chuck 10. Alternatively,the pins may be fixed to the housing and extend through holes in thechuck 10 with sufficient clearance to accommodate the range of motion ofthe chuck 10 with respect to the housing 12. Sealing lands 15 may beprovided around each lift pin hole in order to isolate the lift pin fromthe inner vacuum region of the chuck 10. FIG. 3 also shows anorientation of the flexure mounts 16 to the XY stage, and the positionsensor detectors 18.

[0065]FIG. 4 shows an isometric view of the chuck, similar to FIG. 3,but with the chuck 10 being in an up position, and consequently the pins14 being in a non-extended position. Also shown is one of the radialelectromagnets (i.e., EM) 20, which is shown in FIG. 2 and describedabove.

[0066]FIG. 5A is a cross section view along the line A-A of FIG. 2.FIGS. 5B and 5C show details from FIG. 5A. FIG. 5A shows the chuck 10being in the lowest position relative to the housing 12, with the liftpins 14 thus extending above the surface of the chuck 10 as far aspossible, 7 mm in this embodiment. FIGS. 5A, 5B, and 5C show one of theZ direction actuators, which in this illustrative embodiment consist ofthree distributed actuators, shown in FIG. 6 and described below. The Zactuator in this illustrative embodiment includes a controllable airbearing-guided pneumatic actuator and a VCM. The pneumatic actuator hasa piston 24, which may be made of steel, other magnetic materials or nonmagnetic materials. A piston cap 25, seals one end of the air bearingsleeve and thereby forms a pressure chamber 26. If the piston is made ofa non magnetic material, it may be necessary to provide a separatemagnetic return path. The preferred embodiment uses clean dry air as thepneumatic fluid for air bearings as well as the controllable pneumaticactuator, but other fluid mediums can easily be used. There is apneumatic valve 28 which is operated to control the pressure in thepressure chamber. In a preferred embodiment, the air bearing sleeve 38provides a continuous source of fluid entering the chamber. The valve 28is connected to a remote source of vacuum (not shown) and modulated toobtain the desired chamber pressure. Normally, valve 28 will bepartially open and exhausting to vacuum an amount of fluid that justbalances the fluid entering the chamber from the air bearing sleeve. Thepreferred distance of Z motion of the pistons 24 is at least 6 mm, andmore preferably at least 11 mm. The top surface of the piston 24 pushesagainst a steel pad 32 on the underside of the chuck 10, but isseparated from direct contact with the chuck 10 or with the steel pad 32by a passive air bearing pad 34, made of a porous material, and locatedin a cavity on the top surface of the steel piston 24. The air bearingpad 34 can move in the roll and pitch directions to maintain asubstantially parallel face to the chuck 10 or steel pad 32 because of aball pivot mount 36. The steel pad 32 may be a magnetically preloadedferrous pad region.

[0067] The piston 24 is freely movable in the Z direction withoutsubstantial friction from motion against the housing 12, due to a porouspassive air bearing 38 having an annular air supply 40. In oneembodiment of the invention, the Z air bearing does not have any sealingsurfaces in order to eliminate any friction between the Z actuator andthe housing 12.

[0068] The pneumatic actuator portion of the Z actuators described aboveprovides the static and slowly varying (i.e. low frequency) component ofthe vertical forces necessary for vertical movement for the chuck 10.The only power dissipated within the pneumatic actuator is in the valve28. The valve dissipation is nearly constant regardless of the load thatis being supported. However, the pneumatic actuator is only operableover low frequencies. There is a need for high bandwidth motion that anair actuator may not provide. Thus the Z actuators include at least onevoice coil motor (i.e., a VCM) 42 for each piston 24, located betweenthe piston 24 and a permanent magnet 44, which is located below the airbearing pad 34. The magnet may be magnetized in the vertical directionin a preferred embodiment. The magnet 44 may have a pole piece 46 directthe magnetic flux of the magnet radially across the air gap and into theouter ferromagnetic piston cylinder. The coil 42 of the VCM ispreferably located on the piston cap 25 of the housing 12, and thus theheat-generating portion of the overall VCM is not directly attached tothe chuck. The moving magnet configuration depicted in the figureseliminates the necessity for the VCM coil wires to flex, therebyimproving reliability of the apparatus. It should be noted that acombination of different Z axis motions by each of the Z actuators 22,shown in FIGS. 5A, 5B, and 5C, will result in motion of the wafer topsurface in Z, roll and pitch directions. The Z actuators use the flux ofthe permanent magnets 44 to attract the ferromagnetic steel pads 32,while the air bearing 34 maintains a relatively constant separation fromthe chuck. The variable controlled pressure in the chamber 26 provides Zmotion for low frequency forces, while high frequency forces areprovided by the VCMs 42. There is constant and relatively low heatdissipation due to the use of pressure in chamber 26 to take up staticand slowly varying loads.

[0069]FIG. 5B further shows details of the radial electromagnetactuators discussed in connection with FIG. 2. One of the radial coils20 is shown, wrapped around a pole piece 50, and controllably acting toattract the outer pole piece 52, in conjunction with the other tworadial electromagnet actuators 20 shown in FIG. 2. FIG. 5B also showsdetails of an embodiment of one of the tangential VCM actuators. Thetangential actuators consist of a coil 56, which may have a double loopconfiguration, as is better seen in FIG. 6 (discussed below). The coil56, shown in cross section, operates to rotate the chuck 10 around the Zaxis, known as yaw, using a permanent magnet 60 and pole piece 52 and62. The combination of three (or more) tangential actuators also providesmall amplitude, high frequency XY forces that are not handled by thelimited bandwidth capability of the radial electromagnets.

[0070]FIGS. 7A and 7B show an isometric view of the Z actuator 24 in anextended position, and shows greater detail of the arrangement of thetangential motor elements of the stator52, the coil 56, the permanentmagnet 60 and the inner pole piece 62. Note that the tangential motorstators are normally attached to the chuck 10. The chuck is not shown inFIGS. 7A and 7B even though the chuck would be required to hold thetangential motors stators in the configuration depicted.

[0071]FIG. 6 is an isometric view of the housing 12, with the chuck 10removed, to provide a better view of the relationship of the Z actuators22, the flexure mounts 16, and the encoder read heads 18. FIG. 6 alsoshows three V shaped grooves 66, which are kinematic mounts that providea precise registration between the chuck and housing when the airbearing pistons 22 go to the lowest, or ground state. This “home”position is used to set the initial position of the position sensors 18when power is first applied to the manipulator. FIG. 6 also providesanother view of the tangential VCM motor coil 56.

[0072]FIG. 8 is an isometric view of the chuck 10 in isolation from thehousing, to provide a better view of the relationship between the steelpads 32 on the underside of the chuck. FIG. 8 also shows the ball mounts68 that fit the V shaped grooves 66 of the kinematic mounts. Also shownare holes in the chuck through which the lift pins 14 pass as the chuckis raised or lowered. FIG. 6 also shows the tangential motor statorpiece 52 that is connected to the chuck. The position sensor patternedsurfaces 19 can be seen in FIG. 8. The position sensor arrangement shownin the figure makes use of three 2-dimensional grids as the patternedsurfaces.

[0073]FIGS. 9A and 9B show a different cross section view from FIGS. 5A,5B, and 5C of the housing 12 with the chuck 10 shown in the maximumraised position. There is an other cross sectional view of one of the Zactuators 22. Also shown is a non-magnetic spacer 74 for the Z actuator22 that drives the magnetic flux path through the steel pad 32 on thebottom of the chuck 10. FIGS. 9A and 9B show that the air bearing pad 34may be contained in an inner steel cup 76 that completes the flux pathback to the permanent magnet 44.

[0074]FIGS. 10A and 10B show a top view of the housing 12 shown with thechuck 10 removed that shows the relative positions of the parts of the Zactuator 22. The Z actuator includes a cylindrical air bearing sleeve38, a steel piston 24 separated radially by a non-magnetic spacer 74from an inner steel cup 76. Inside the inner steel cup is the airbearing pad 34, which is mounted on the ball pivot 36 (not shown inFIGS. 10A and 10B) to maintain planarity with the bottom of the chuck(not shown in FIGS. 10A and 10B). A distribution of the Z actuators 22,the tangential actuators 56 and the radial electromagnet actuators 50 isshown in FIGS. 10A and 10B. Note that the Z actuators may help controlmotion in the Z, roll and pitch directions, and that a relatively lowand a constant power consumption is due to the use of pneumatics.

[0075]FIG. 11 is an isometric view of an alternative embodiment of theinvention. Some intended applications for the substrate manipulator haveless stringent precision requirements and greater cost sensitivity. Thealternative embodiment shown in FIGS. 11-19 provide four degrees offreedom of control of the chuck position with respect to the housing.The alternative embodiment eliminates the radial electromagnet actuatorspreviously described and makes use of lower cost “linear” positionsensors. FIG. 11 shows a housing 12 and chucktop 10 with lift pins 14protruding through the chucktop surface. The housing 12 would normallybe attached directly to the top surface of an XY stage. FIG. 12 showsthe alternative embodiment in the raised position.

[0076]FIG. 13 is an exploded isometric view of the alternativeembodiment. A hub assembly 90 is driven with respect to the housing 12and in turn carries the three Z actuators that drive the chuck in Z,roll and pitch with respect to the hub. The hub is restricted to performonly a yaw motion by a combination of 3 magnetically preloaded airbearing pads and 2 magnetically preloaded cam followers 88. FIG. 14shows these items in greater detail. The air bearing pad 82 ismagnetically attracted against a ferromagnetic surface 78 that isattached to the housing 12. However, the pressure of the air bearingensures that a small gap, usually less than 10 microns, exists betweenthe surface of the air bearing pad 82 and the ferromagnetic surface 78.A pair of magnets 80, magnetized in the vertical direction provides theattractive force that preloads the air bearing against the ferromagneticsurface 78. FIG. 15 is a view of the ferromagnetic surface 78 attachedto the underside of the housing 12. The ferromagnetic surface 78includes a flux focusing region that directs the magnetic flux ofmagnets 80 radially inward across tangential motor coils 56 and intoinner stator 79. The ferromagnetic stator piece 79 completes themagnetic path for the flux back to the underside of magnets 80. Notethat magnets 80 are arranged with either both “N” surfaces up or both“S” surfaces facing up.

[0077]FIG. 14 shows cam follower 88 attached to the hub assembly. Twocam followers are provided as shown in FIG. 13 and that ride along theinner circumference of housing 12. The cam followers are magneticallypreloaded against the inner surface of the housing. The magnetic fluxcrossing the tangential motor coils 56 also creates a radial force thatacts between the hub 90 and housing 12. Normally, the symmetry of thethree tangential actuators would ensure that the radial forcessubstantially cancel each other. With perfect symmetry, there would beno net radial force with which to preload the cam followers.

[0078] The symmetry of the three tangential actuators is deliberatelyupset by altering the dimensions of the flux focusing face of oneferromagnetic pad 78 relative to the other two pads. A small change inthe cross-sectional area of the innermost radial edge of the pad 78produces a large change in the radial force but only a minimal change inthe force constant of the tangential motor. The unbalanced radial forcescreate the preload force necessary to ensure that the cam followersremain in rolling contact with the inner surface of the housing duringnormal expected stage motions.

[0079] The three tangential motors may be driven as if they are a singlemotor. Each motor produces a tangential force acting between the housingand hub assembly. The three motors may produce a yaw moment about the Zaxis in order to rotate the hub assembly 90.

[0080] The position of the hub assembly with respect to the housing ismeasured using a position sensor. Numerous encoder configurations knownto those skilled in the art may be used to measure the hub rotation. Anencoder read head 84 is shown in FIG. 15 attached to the inner surfaceof the housing. A small section of flexible sensor substrate 86 may beadhesively attached to one of the inner stator plates 79 as shown FIG.13 and in cross section in FIGS. 17A, 17B, 17C, 17D, and 17E. The samestyle of encoder and substrate may be used between the hub assembly 90and the chucktop 10 to measure the chuck position with respect to thehub in the Z, roll and pitch directions. The encoder 84 may be mountedto the hub as shown in detail in FIG. 16B. The encoder substrate 86 maybe attached to a dowel pin and bonded into the underside of the chucktopas shown in FIG. 16B and in FIGS. 18A, 18B, and 18C.

[0081] The chucktop is driven with respect to the hub assembly in the Z,roll and pitch directions using three Z actuator assemblies 22. In thealternative embodiment the three Z actuators incorporate magneticallypreloaded kinematic mounts consisting of a ball 92 fixed in the top ofthe piston 24 that makes contact with Vee-block 96 fixed in theunderside of the chucktop. Alternative configurations of kinematiccouplings may be obvious to one skilled in the art. Preload magnets 96are bonded to the ferromagnetic Vee-blocks 94 with opposing polarity.The kinematic ball 92 is preferably ceramic or similar non-magneticmaterial in order to avoid fretting corrosion with the ferromagneticVee-blocks. Alternatively, the flux from the permanent magnet 44 used inthe voice coil motor portion of the Z actuator could be exploited tocreate the magnetic preload. However, the magnitude of force required toachieve sufficient preload to withstand anticipated stage motions may besubstantially lower than the force that was required in the previouslydescribed embodiment in order to properly preload the air-bearing pad.The substantially smaller preload may be better obtained by use ofrelatively small permanent magnets 96 as shown in FIGS. 18B and 18C.

[0082] The home position for the three Z position sensors 84 may beestablished using a kinematic mount between the chucktop 10 and the hubassembly 90 that may be engaged at the extreme bottom of stroke of the Zactuators. A preferred embodiment of the kinematic mount may be realizedusing three balls 68 that make contact with three horizontal surfaces ofthe hub assembly. The balls 68 are visible in FIGS. 18B and 18C and incross section in FIGS. 17A, 17B, 17C, 17D, and 17E. The balls 68 makecontact with the top surface of the inner stator plate 79. The threeballs in contact with three flat surface provide constraints for thethree degrees of freedom that exist between the hub and the chucktop.Alternatively, a switch or other sensor may be used to provide a homesensor for the encoders. The kinematic constraint shown in FIGS. 17 and18 has the advantage of simplicity, low power and low cost versusalternative home sensing solutions.

[0083] A controller is used to control the chuck system with respect tothe housing. The control system converts between various coordinatesystems, such as Cartesian and polar. The control system manipulatesdata from various points of view, such as the user, the substrate beinghandled, and the significant points of the tools in which the chuck isbeing used, such as the center of an optical system in an imagingsystem. The user may initiate (i.e., command) motions of the chuck in acoordinate system that may be considered as if the chuck were “embedded”in the “focal plane” of the imaging system tool. A primary purpose ofthe chuck in the embodiment illustrated herein is to bring a substrateinto alignment with the imaging system tool. Sensors in the imagingsystem tool may observe deviations from alignment (for example, out offocus, or out of the focal plane). The deviations from the desiredposition and attitude of the wafer with respect to the alignment toolresults in the tool (i.e., the user in this illustrative example)issuing commands to the chuck controller to compensate for the observeddeviation. There are many well known coordinate systems that could beused for moving the handler. A preferred system is a Cartesiancoordinate system that is parallel to the focal plane of the tool andaligned with the tool's definition of X and Y. The Z=0 plane of thecoordinate system lies in the focal plane of the image of the tool. Theadvantages of selecting such a “tool-centered” coordinate systemincludes the convenience of expressing rotations to the chuck withoutintroducing XY translations to the flat stage upon which the chuck andits housing moves.

[0084] Thus, in a preferred embodiment, the control system may use a“body-centered” coordinate system that is distinct from the usercoordinate system. The body-centered coordinate system is preferred dueto the well-known principle that the equations of motion of a rigid bodyare decoupled when the equations are expressed in a coordinate systemlocated at the center of mass of the body and oriented along theprinciple directions of the body. Thus, the six degrees of freedom(i.e., 6-DOF) in the body-centered coordinate system of thisillustrative embodiment enables the control system to be comprised ofsix 1-DOF controllers. Similarly, the three degrees of freedom in thebody-centered coordinate system of this illustrative embodiment enablesthe control system to be comprised of three 1-DOF controllers and thefour degrees of freedom in the body-centered coordinate system of thisillustrative embodiment enables the control system to be comprised offour 1-DOF controllers. This is an advantage when the controllers arenon-linear, since saturation of the actuators and limits on maximumvelocity may force the controllers to become non-linear. It is fareasier to apply non-linear control to a 1 DOF system than to try tohandle non-linearity inside a strongly coupled multi-DOF controller.

[0085] The controller in this illustrative embodiment of the inventioncontrols the chuck (or handler) in what are known as the six degrees offreedom (i.e., 6-DOF), which are the three dimensions of normal space,X, Y and Z, and the rotations around those three axis, known as roll,pitch and yaw respectively. The six directions result in what are knownas six body forces that are applied to the handler to obtain themotions. In addition, the motions may be either high frequency motionsor low frequency motions. The six body forces in the handler or chuckare produced through the control of a combination of 12 currents. Thereare three Z voice coil magnet (i.e., VCM) currents, three radialelectromagnet (i.e., EM) currents, three tangential VCM currents, andthree currents in the valves that regulate the pressure in the Zcylinders (VSO current). The body force controller generates the bestset of the twelve control signals in order to efficiently deliver theoverall body force needed.

[0086] Alternative embodiments shown herein control the chuck in eitherthree degrees of freedom or four degrees of freedom, which is astraight-forward adaptation of the six DOF system illustrated herein.

[0087] In general, a set of control signals includes the use of thepneumatic systems to maintain a low power dissipation for relativelyconstant loads, the use of the EM actuators rather than the VCMactuators when possible to keep power dissipation as low as possible,the separation and combination of high frequency from low frequencyactuators, and the use of magnetic preloads and passive air bearings inconjunction with use of differential mode operation when possible tokeep the power dissipation level as constant as possible. The followingdiscussion is an illustrative example of how to use the above notedgeneral features in a control system that achieves the goals of thepresent arrangement of highly accurate, fast and reproducible movementof a substrate with low power dissipation and constant powerdissipation.

[0088] The chuck manipulator controller is an element of the controlsystem. The chuck manipulator controller provides the function ofclosing a position servo loop and responding to user position commands.The controller follows the conventional form of a state-space controllerwith full-state observer. Those skilled in the art will recognize inFIG. 19 the role of the block labeled “bPos_Ctrl” as the block in whichthe controller is implemented. The block labeled “bObserver” providesthe estimate of the state variables in response to measured position(from 6 position sensors) and (known) force applied to the manipulator(“bF_actual”).

[0089] The block labeled “User_J_bPos” is used to transform from theuser coordinate system to the body-centered coordinate system. Thetransformation is in general non-linear due to the presence ofrotations. However, the range of rotation in the intended application ofthe manipulator allows for a linearized transformation based on smallangle assumptions. It is important to remember that the primaryapplication for the manipulator is for aligning a substrate to a tool.In this situation, any inaccuracy induced by the linear approximation tothe non-linear coordinate transformation (however small) is corrected bythe closed loop auto-focus, auto-planarize and auto-align features ofthe tool.

[0090] The nomenclature used in FIG. 19 follows a simple convention. Alowercase “b” prefix indicates that the signal (“bF_actual” for example)is referenced to the “body” coordinate system. Note that there aremultiple coordinate systems present in the manipulator. The toolcommands the manipulator in the “user” coordinate system. The encoderssense the chuck position with respect to the housing in the “encodercoordinate system”. The Z actuators apply bZ, bRoll and bPitch forces(moments) by forming sums and differences along the [Z1, Z2, Z3] vectordirections. The tangential motors apply bFx, bFy and bYaw forces(moments) through sums and differences along the [T1, T2, T3] vectors.

[0091] The radial electromagnets apply radial forces along the [R1, R2,R3] directions. The “R” directions nominally span the 2D Cartesiandirections [bX, bY]. Since 3 “R” directions map into 2 body directionsthere is some redundancy. This redundancy is exploited to compensate forthe inability of the radial EM motors to provide negative force(electo-magnets can only “pull”, they cannot “push”). The radial EMmotors do not pull directly in-line with the body center of mass (CG).The radial motors produce a slight un-intended bRoll and bPitch momentin the course of producing the desired bFx and bFy forces. This issue,as well as the partitioning of forces between the (slow, efficient)radial EM motors and the (fast, less-efficient) tangential motors, ishandled in the block labeled “bForce_Control”.

[0092] Six body forces in the manipulator are produced through thecontrol of a combination of 12 currents: (3) Z VCM currents, (3) RadialEM currents, (3) Tangential VCM currents and (3) currents in the valvesthat regulate the pressure in the Z cylinders (VSO current). Thefunction of the body force controller is to generate the best set of 12control signals in order to efficiently deliver the body force that isrequested by the control law (b_F_control).

[0093]FIG. 20 reveals how the 12 currents are determined in response toa 6 DOF bForce command. The 6D command for a body force appears in theupper left corner of the figure (“b_Fc”). The 6D body force is “issued”to the radial EM controller. Only a subset of the body force can begenerated by the radial EM controller. Also, the modest bandwidth of theradial EM amplifiers (300 Hz, for instance) will prevent the radial EMcontroller from producing all of the requested force. Finally, since theradial EM motors do not act directly at the body CG, they will producemoments about the body X (Roll) and Y (Pitch) axes.

[0094] The radial EM controllers produce a signal (“b_Fr”) that is theactual body force produced by the radial EM. This signal will containsome residual bFx and bFy components due to the limited bandwidth of theradial EM. Also, it will contain the original bRoll and bPitch momentsalong with the algebraic addition of the Roll and Pitch moments“inadvertently” created by the radial EM motors. Note that the net bRolland bMoments may be larger or smaller in magnitude than the initial b_Fccommand due to the effect of the radial EM motors.

[0095] The net body force produced by the radial EM motors is subtractedfrom the original body force request. The remaining body force (whichwill typically contain a small component in the X and Y directions andrelatively larger components in the Z, Roll, Pitch and Yaw directions)serves as the command to the Tangential VCM controller and the Zactuator controller.

[0096] In the general case, the residual body force (after the radial EMforces are subtracted from the initial body force) would be applied tothe tangential motors, the net body force computed and a new residualbody force computed and used as the input to the Z actuators. Thetangential motors and the Z actuators are known a priori to be mutuallyorthogonal. There is no component of the 6D body force that can beproduced by the tangential motors that could also be produced by the Zactuators (and vice versa). In this case, there is no loss of generalityto apply the residual force (after the radial EM controller) to both theTangential VCM controller and Z Actuator Controller. The transformationswithin each controller that transform the 6D body force vector into therespective 3-tangential directions and 3-Z-actuator directions ensurethat there is no coupling between the tangential and Z-actuatordirections. It should also be mentioned that the 3-tangential and3-Z-actuator directions collectively span 6-space. Thus the combinationof “T” and “Z” 3D force controllers is sufficient to span the required6-DOF space encompassed by the body force vector. However, there stillremain issues of actuator saturation and finite bandwidth within theZ-actuators. These issues will be discussed momentarily.

[0097] The effect of actuator saturation and finite bandwidth is tolimit the available net body force produced to something less than theinitial request. The net actual body force is computed by the vector sumof the body forces produced by the radial, tangential and Z-actuators.This actual body force is used in the state estimator as the “known”input to the plant model (well known to those skilled in the art). Theactual body force will be identical to the b_F_control signal thatoriginated from the control law under most circumstances. If an actuatoris saturated and therefore unable to deliver the necessary force, theobserver will be better able to estimate the proper state variables dueto the use of the saturated body force signal (which is a betterindication of the known force applied to the manipulator than theoriginal).

[0098] The radial force controller is shown in FIG. 21. The (6D) bodyforce command appears at the left of the figure (“bForce”). Therelationship between force and current in an electro-magnet is afunction of motor parameters, coil current and the air gap. The gapcalculation is a function of the body position (suitably transformed)and the initial gap that is present when the body position is “zero”.

[0099] The radial EM motors are operated at a nominal bias force. Thenominal bias serves to minimize fluctuations in power dissipationamongst the radial motors. Electro-magnet actuators are only able togenerate an attractive force, which is considered positive in thisillustrative embodiment. To generate a negative force, the opposing pairof EM actuators must each generate −1 times the “negative” force (whichbecomes a positive force for the opposing pair of EM motors). It helpsto bear in mind that the three radial EM motors act along radialdirections equally spaced around the Z axis of the chuck (i.e., thecenter of the chucktop). A force of “−1” along the direction of a firstone of the radial EM motors cannot be produced directly by the firstradial EM motor. Rather, the second and third radial EM motors must eachproduce +1 units of force in their respective directions. The vector sumof +1 along each of the directions of the second and third radial EMmotors sums to a net force of “−1” along the direction of the firstradial EM motor. The power dissipated in an electromagnet is directlyproportional to the force generated (in contrast to a VCM where thepower is proportional to the square of the force). The radial EM motorwill dissipate constant total power as long as the sum of the radialforces is constant.

[0100] In the absence of a bias force, the nominal force (and thereforepower dissipated) amongst the three radial EM motors is zero. As thestage that supports the manipulator moves, the radial EM motors willgenerate forces (and thereby power dissipation) in response to the stagemotion. There will be a net change in total power dissipation betweenwhen the stage is resting (dissipation=zero) and when the stage isoperating (non zero power).

[0101] Consider the addition of a bias force of 10 newtons acting alongeach radial actuator. The vector sum of the radial bias force produceszero net body force (the bias forces “lock-in” a strain within thechucktop). There will be non-zero quiescent power dissipation (evenwhile the stage is not moving) in proportion to 10+10+10=30N.

[0102] Consider the case where, in response to stage acceleration, a netbody force of −1 newtons in the Y direction is required. Reducing thebias force of a first one of the radial EM motors (aligned along +Ydirection) by 1 newton could produce the desired net force of −1 newtonin the Y direction. However, the total power dissipation would now beproportional to 9+10+10=29 newtons. The same net force could also beproduced by driving the first radial EM motor at (10⅔) newtons and thesecond and third radial EM motors at (10+⅓) newtons each. The net forcein the Y direction is still (−⅔−0.5(⅓+⅓))=−1. The total power dissipatedwould be proportional to (10−⅔)+(10+⅓)+(10+⅓)=30 newtons.

[0103] To summarize, it is possible to exploit the redundancy inherentin the 3 radial EM motors (that collectively span only 2D space—X,Y) toachieve a 2^(nd) objective beyond simply generating a net force. In thecase of the wafer manipulator, the additional objective is constantpower dissipation. This capability ensures that the manipulator willalways dissipate the same total power regardless of the stage dutycycle. It is preferable in some instances to dissipate a constant powerthan to dissipate power that varies in response to stage duty cycle.While any power dissipation is undesirable, the precision of a stage isfar more tolerant of constant (albeit modest) power dissipation than itis to varying power dissipation. Note that the choice of bias forcecould be made in anticipation of the range of maximum force required,for example due to expected stage acceleration. A bias of ION isreasonable for the typical mass and stage acceleration expected of awide range of applications, such as in the semiconductor fabricationarea. The total power dissipated from the three radial EM motorsoperating at a bias of 10N may be less than 1.2 watts.

[0104] The cluster of blocks at the middle left of FIG. 21 isresponsible for introducing a force bias and ensuring that the forcesupplied by a single EM motor is non-negative. If an EM motor isrequired to produce an incremental negative force larger than the forcebias, the EM motor force is limited to zero and the balance is producedby the opposing pair of EM motors (½ each). Under these conditions, thetotal net force is achieved, but there is an attendant increase in powerdissipation. This condition may not be reached if the bias force is setproperly.

[0105] Once the desired force for each radial EM motor is determined,the current required to generate the force can be easily determined, asshown in FIG. 22. The force produced by an EM motor is simply:F=C*(I/gap)². This is easily inverted to determine the desired coilcurrent.

[0106] The EM controller must also produce an estimate of the actualforce that is generated. The actual force may differ from the desiredforce due to the finite bandwidth of the EM amplifiers. The first ordermodel of the radial EM motor response to a current command provides areasonable estimate of the actual current in the EM coil. Atransconductance amplifier with well-known dynamic characteristicsproduces the actual coil current. Using an open loop model of theamplifier/coil dynamics may be preferred over the alternative ofmeasuring the actual coil current since any measurement of coil currentmay be subject to quantization effects due to the use of an A/Dconverter. The noise produced by the quantization may be introduced intothe body force calculation, which in turn could influence the current inthe tangential VCM motors. Since the tangential VCM motors are operatedat high-bandwidth, the use of measured coil current could degrade thenoise performance of the manipulator. In this application, some error inpredicted radial force (due to slight uncertainty in the estimated motorforce) is preferable to injecting high frequency noise into the system.

[0107] Another task for the radial EM force controller is to estimatethe net body force actually produced in response to the command. The netbody force may be due to the (bandwidth limited) actuator force as wellas the effect of the force being applied at a location other than the CGof the body. Any non-zero net force in the XY plane due to the radial EMmotors may also produce a Roll or Pitch moment. These moments arecompensated by the Z-actuators in order to achieve the net 6D body forceneeded for the decoupled control algorithm.

[0108]FIG. 23 shows that the tangential motors produce force in directproportion to the coil current: F_(t)=K_(t)*I. However, the motors maybe subject to saturation. The desired force may be limited to themaximum tangential motor force under certain circumstances. The actual(possibly limited) force may be expressed in the body coordinate systemand summed within the overall body force controller to determine anactual net body force.

[0109]FIG. 24 illustrates how the force required of each Z actuator isapportioned between the pressure actuator and the integral voice coilmotor. The desired force to be delivered from a single Z actuatorappears as the input “Fp_Cmnd” at the upper left of the figure. Theforce request is converted into an equivalent pressure by dividing bythe cross-sectional area of the piston (“Inv_Area”). This signal is usedas the command for the pressure controller.

[0110] The operation of the pressure controller will now be explained.At any instant in time, the net force delivered by the Z actuator is thesum of the pressure force due to the pressure in the cylinder (i.e.,pressure times cylinder cross-sectional area) and the voice coil forcedue to any current in the VCM windings (i.e., current “I” times motorconstant “Kf”). The request for Z force is first applied to the pressurecontroller. The pressure controller may attempt to deliver all of therequired force. The actual force due to the pressure, which may not beequal to the requested force, is subtracted from the original forcerequest. The voice coil motor may be used to provide the remainder ofthe required force. If a large, rapidly changing force is requested, thepressure controller may not be able to change the pressure sufficientlyfast to follow the request and a large “force error” may be applied tothe VCM. The VCM, however, is subject to saturation. The VCM forcelimits may be detected in the controller before the physical actuator issent into saturation.

[0111] The pressure controller may be implemented in a classicalproportional plus integral (“PI”) configuration. The pole correspondingto the integral term of the controller determines the range offrequencies over which the pressure servo will provide the requestedforce. At DC, the integral action provides that there is no error in thepressure servo. This implies that (at DC) there is no force supplied bythe VCM. At low frequencies (up to 40 Hz, for example), the majority ofthe force request may be satisfied by the pressure servo and very littleforce may be delivered by the VCM.

[0112] When large step movements are required in the Z direction, theremay be a sudden, large step in the force command. The pressurecontroller may not be able to respond instantly to the command and themajority of the initial force step may be applied to the VCM motor. Ifit is sufficiently large, the VCM motor may saturate.

[0113] Within a few milliseconds, the pressure in the cylinder willstart to change in the appropriate direction to satisfy the request andthe current in the VCM motor will decay rapidly. Since there is no DCcurrent in the VCM (due to integral action in the pressure controller)and the large currents encountered during steps decay rapidly, there isnegligible average power dissipation in the VCM coils. If the Zactuators were required to make large, rapid and frequent force changes,the VCM motors would experience heating. However, this is not thetypical case for the intended application of the wafer manipulatormechanism.

[0114] Even if large external loads are applied to the stage (whichrequire corresponding large DC forces in the Z actuators), the entireload is supplied by the pressure controller after a small fraction of asecond. Wafer probing is an application with large quasi-DC loads. Whenthe wafer is brought into contact with the probe pins, a large force maybe applied to the chuck. Probe forces can be very large relative to thegravity force due to the chuck and substrate mass. Somenon-semiconductor applications may involve substrate masses that are asignificant fraction of the chucktop mass. A large quasi-DC force(change) is required when such a substrate is loaded/unloaded from thestage. These large quasi-static loads are easily handled by themanipulator without excessive heating due to the apportioning of theforce between the high-frequency small force capabilities of the VCM andthe low-frequency, large force capabilities of the pressure cylinder.

[0115] Note the similarity between the bandwidth partitioning betweenthe radial and tangential motors and the partitioning within the Zactuator. The large amplitude, efficient but lower bandwidth actuator isused to satisfy the force request. Only the residual force that can notbe satisfied by the bandwidth limitations of the efficient actuator isleft for the high-bandwidth, less-efficient VCM motors. The Z-actuator,however, incorporates integral control to ensure that there is never anyDC force required of the Z VCM. The Z actuators are required to supportlarge DC loads. Thus it is very important to ensure that the controllerhas no “DC-path” from the command to the VCM. The use of integralcontrol in the pressure servo provides such a condition.

[0116] The presence of the low pass filter (“R EM Bandwidth” block) inthe R EM force controller shown in FIG. 21 provides a function similarto the use of integral control in the pressure controller. The output ofthe low pass filter is analogous to the input at low frequencies. Thus,the force estimate of the radial EM may match the force request at lowfrequencies. This provides that there will be not net force applied tothe tangential VCM motors at DC. The response of the pressure controllerto a pressure command is more difficult to accurately predict than theresponse of a transconductance amplifier to a current command. For thesereasons, it may be preferable to measure pressure directly and employintegral control in the pressure controller to eliminate any DC forcesfrom the Z VCM motors.

[0117]FIG. 25 depicts the arrangement of the pressure controller. Thepressure controller may be implemented in a conventional PI (i.e.,proportional+integral) configuration with an observer used to provide an“estimate” of the actual cylinder pressure. This is consideredconventional practice by those skilled in the art of control systems.Since a pressure sensor is included in the system, one might questionthe reason behind the use of a pressure observer to estimate pressure.The dynamic performance of the pressure controller is little affected bythe use of estimated pressure rather than measured pressure (i.e., usingthe pressure sensor). However, the raw pressure measurement includes thesignal of interest, the cylinder pressure, as well as noise. The noiseon the pressure sensor signal is due to ever-present electrical noise aswell as quantization effects due to finite precision A/D (i.e., analogto digital) conversion of the analog pressure sensor signal. These noisesources lie outside of the effective bandwidth of the pressurecontroller. However, since the pressure signal is used in determiningthe (high bandwidth) VCM force, any noise on the pressure signal will beintroduced into the system. At the nanometer level of precision that isrequired of the manipulator, such noise would compromise performance.

[0118] The estimator (also know as an observer), provides a veryeffective filter to limit the spectrum of noise that would otherwise beintroduced into the VCM motors. The estimator is a convenient locationwhere the non-linear behavior of the pressure cylinder dynamics can behandled as well.

[0119]FIG. 26 depicts the pressure observer. The form of the observerfollows conventional practice for a full-state observer (i.e., a 1 statevariable) with a disturbance estimate. The observer makes use of asimplified equation for the gas properties related to the cylinder. Thefull equations for the gas properties are computationally “expensive”and not needed over the modest range of anticipated pressures. Thelinear version of the equations is shown in the figure.

[0120] There are 3 factors that determine the rate of change of pressure(“Pdot”) in the cylinder. Mass flow from the air bearing is continuallytrying to raise the pressure in the cylinder. Mass flow from the valveis modulated to balance bearing mass flow. Also, if the cylinder ismoving, then the pressure in the cylinder would nominally increase ordecrease depending upon whether the volume were decreasing orincreasing. This effect is captured in the observer with a term thatmultiplies the actuator velocity (“vel”). The relation between mass flowand pressure change is a function of the mass properties (“kRT”) and thecylinder volume. The cylinder volume is a function of the position ofactuator and the amount of “dead space” in the cylinder.

[0121] The “open loop” computed rate of pressure change is integrated(“est Pressure”) to provide an estimate of pressure. This in turn iscompared with the measured pressure to form an error term. The error inpressure acts through the observer gain (“Obs_gain_Pc”) to ensure thatover the frequency range of interest, the estimate of pressure is anaccurate representation of the actual pressure.

[0122] Note however, that if the open loop estimate of Pdot contained aslight error, a non-zero constant error in estimated pressure would berequired in order to provide a “0” at the input to the “Est Pressure”integrator. This would constitute an error in the force estimate for theforce controller and lead to a reduction in system performance.

[0123] Slight variations in the pressure supplied to the air bearing(due, for instance to the normal level of compliance in typicalmechanical pressure regulators) may lead to slight variations in theactual mass flow entering the cylinder. Other variations are anticipateddue to changes in the piston to cylinder gap with temperature and age aswell as aging in the valve.

[0124] These effects can be eliminated from the pressure estimator bythe well known use of a disturbance estimator in addition to thepressure estimate. The block labeled “Est Pdot Disturbance” in thefigure above provides the necessary correction. Note that the input tothe disturbance integrator comes directly from the pressure error. Thepersistence of any error between measured pressure and estimatedpressure will drive the disturbance estimator so as to remove the error.Through the introduction of the disturbance estimator, the estimatedpressure will have no DC or low frequency deviation from the measuredpressure. This method ensures a high degree of accuracy in the forceestimated from the pressure cylinder, which improves the overallperformance of the Z actuator.

[0125] The benefit of using the observer to estimate the cylinderpressure and thereby reduce noise propagation into the high-bandwidthVCM, is realized by setting the poles of the pressure observer at arelatively low frequency (100 Hz, for instance). The observer functions“open loop” at higher frequencies based on the known command sent to thevalve and the measured position and velocity of the cylinder. Thedisturbance estimator corrects the expected errors in the “open loop”model at low frequencies. The result is a “clean” estimate of the actualcylinder pressure covering a wide dynamic range.

[0126] While the invention has been disclosed in connection with variousembodiments, modifications thereon will be readily apparent to thoseskilled in the art. Accordingly, the spirit and scope of the inventionis set forth in the following claims.

1-65. (canceled)
 66. An actuator that manipulates an object, comprising:a housing; at least three tangentially acting voice coil motors disposedto move the object relative to the housing in a yaw direction; at leastthree radially acting electromagnet actuators maintaining asubstantially constant gap between the object and a housing independentof motion in the yaw direction; and a control system that supplies theobject with large low-frequency forces in a plane of the object usingthe radially acting electromagnet actuators, and supplies low amplitudehigh-frequency forces in the plane of the object and provides forces formotion in the yaw direction using the tangential voice coil motors.67-69. (canceled)
 70. An actuator, according to claim 66, wherein anincrease in the force level of one of the plurality of radial actuatorsis counterbalanced by a reduction in the force level of at least oneother of the radial actuators so that the power consumed by all of theradial actuators together is substantially constant.